- Title
- Decoding specific ion effects: polymer brushes in complex electrolytes
- Creator
- Robertson, Hayden
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2024
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Polymer brushes are densely end-tethered polymer arrays. When composed of stimuli-responsive polymers, brushes form a foundation for smart interfaces that respond to a given set of environmental factors, such as temperature, pH, or ionic strength. For example, thermoresponsive polymers such as poly(N-isopropylacrylamide) (PNIPAM) or poly(oligoethylene glycol methacrylate) (POEGMA), undergo a phase transition from well- to poorly-solvated over a broad temperature range. These smart coatings modify interfacial properties such as surface charge, lubricity, and adhesion, thereby improving the deployment of the underlying material in otherwise challenging media. Polymer brushes offer a valuable platform for investigating diverse physicochemical phenomena, such as specific ion effects (SIE), as their behaviour can be comprehensively characterised via a suite of techniques. SIE relate to any phenomena that depend on the ion identity rather than merely charge or concentration. That is, in the presence of different ions, the nanostructure of responsive polymer brush coatings can be tuned to dramatically alter, for example, a system’s wettability. Historically, the manifestation of SIE in simple aqueous electrolytes containing a single halogen-based anion has been well studied; for the most part, ion specificity adheres to the Hofmeister series. However, in more natural environments, such as multi-salt electrolytes, non-aqueous systems or electrolytes composed of biologically relevant ions, these complex SIE are less well studied. Consequently, there is no all-inclusive theory for SIE, and given the ubiquitous nature of these more complex electrolytes, understanding the interplay of different ions and their solvation is imperative for real-world applications. Herein, stimuli-responsive polymer brushes are used as exemplar systems to probe SIE in complex environments. Importantly, a key benefit of examining the brush regime is that the concentration of polymer remains invariant across all measurements, minimising uncertainty. Moreover, the behaviour and conformation of the brush can be examined irrespective of the polymer solvation state. The complementarity of interfacial characterisation techniques, such as neutron reflectometry (NR) and spectroscopic ellipsometry (SE), is key to understanding the intricate brush behaviour. However, due to the inherent diffuse nature of these brushes, the robust analysis of data relating to these coatings has proven difficult. First, novel methodologies are developed and key instrument advances are discussed in relation to NR and SE. These key advancements are then utilised to investigate polymer brushes in various complex environments. The influence of mixed electrolytes on the behaviour of a POEGMA statistical copolymer is then examined. This investigation identified concentration-dependent interactions (either cooperative or antagonistic) between the ions in the mixed electrolyte. Further employing these methodologies, a PNIPAM brush in the presence of biologically relevant short-chain fatty acids anions is studied. Intriguingly, this study revealed a chain length dependence on the ability of an anion to salt-out PNIPAM, indirectly examining hydrophobic hydration. The final solely aqueous study probed the behaviour of a poly(2(methacryloyloxy)ethyl)trimethylammonium (PMETAC) brush in highly concentrated electrolytes. Notably, the behaviour of the PMETAC brush in these highly concentrated electrolytes deviates from that which can be explained by classical electrolyte theory, exhibiting underscreening via re-entrant swelling. Lastly, the role of solvent molecular structure on SIE and polymer solvation is explored, specifically focusing on the effects of dimethylsulfoxide (DMSO) mixtures with water. In water-rich regimes, PNIPAM exhibits a lower critical solution temperature (LCST) in conjunction with a forward Hofmeister series. However, in DMSO-rich regimes, PNIPAM exhibits upper critical solution temperature (UCST) behaviour where the direction of ion influence is dependent on electrolyte concentration. Consequently, a theoretical anion that imparts no impact on the swelling of a polymer brush agnostic of solvent composition was noted. This work details the development of new approaches and software tools to characterise polymer brushes in complex environments; such as the open-source Python package, refellips. By providing a set of detailed analyses, this research seeks to enhance the application of polymer brushes through detailed interrogation with a view to increased control of coating behaviour. These advancements also aim to elucidate the intricate role between the solvent, substrate (polymer) and solute (ion) in an effort to decode the fundamental origins encompassing SIE.
- Subject
- polymer brush; specific ion effects; surface chemistry; atomic force microscopy; re-entrant swelling; cononsolvency; thesis by publication; Hofmeister; ellipsometry; neutron reflectometry; underscreening; non-aqueous electrolytes; polymers; polymer conformation; quartz crystal microbalance with dissipation monitoring
- Identifier
- http://hdl.handle.net/1959.13/1499977
- Identifier
- uon:54820
- Rights
- Copyright 2024 Hayden Robertson
- Language
- eng
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| Thumbnail | File | Description | Size | Format | |||
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| View Details Download | ATTACHMENT01 | Thesis | 133 MB | Adobe Acrobat PDF | View Details Download | ||
| View Details Download | ATTACHMENT02 | Abstract | 326 KB | Adobe Acrobat PDF | View Details Download |